Two years ago, while visiting his family in the Russian region of Dagestan, Tamerlan Tsarnaev, the prime suspect in last week’s Boston Marathon bombings, was flagged as a potential extremist by Russian security services. The only evidence they had were his regular visits to a mosque that gets more than its share of attention from police. Since its construction in 2000, the mosque’s broad, emerald-colored dome has been the center of the region’s Salafi community, which adheres to a more orthodox brand of Islam and, over the years, has been a hangout for men killed in shootouts with Russia’s counterterrorism forces.

According to a source close to the Russian security services who specializes in religious radicalism, Tsarnaev attended services at the mosque on Kotrova Street during both of the extended visits he made to Dagestan over the past two years. That is why Russia’s Federal Security Service, the agency better known as the FSB, sent a warning to the FBI in 2011 to be aware of Tsarnaev’s possible links to extremism.

In a statement on April 19, the FBI said it had received information from an unidentified “foreign government” that Tsarnaev was “a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared … to join unspecified underground groups.” In response, the FBI said it interviewed Tsarnaev and checked its records for relevant information, but “did not find any terrorism activity, domestic or foreign.”

According to the source in the regional capital of Makhachkala, who spoke to TIME on Monday, Tsarnaev was monitored by Russian counterterrorism forces for at least one month in 2011 and throughout his six-month stay in Dagestan last year. “There wasn’t enough time [in 2011] to come to any conclusions about the extent of his involvement [in Islamist extremism],” the source says, asking to remain anonymous due to the sensitivity of the matter. “So we asked our American colleagues to follow up.”

His account was corroborated by a source close to the FSB in the city of Khasavyurt, the second largest city in Dagestan, who spoke to TIME on Sunday. “It didn’t take much for him to raise suspicion,” the Khasavyurt source said of Tsarnaev, also insisting on anonymity. “Showing up at a Salafi mosque was enough.”

There is no indication that Tsarnaev, who was killed in a standoff with Boston police on Friday, was instructed or pushed toward committing any terrorist acts during his visits to the mosque on Kotrova Street. The vast majority of the mosque’s congregants likely have no connection to the region’s extremist activity, and more moderate Muslims regularly attend services there. Both of TIME’s sources said the Russian security services never observed Tsarnaev make contact with any of the known insurgent leaders or suspected terrorists who operate in Dagestan. But the sermons he heard at the mosque might have contributed to his gradual radicalization, the sources said. “The idea that America and Israel are the axis of evil is pretty typical there. He would have heard some of that,” said the source in Makhachkala. He added, however, that the extremist videos he watched online could also have been an important factor.

On his YouTube channel, which he opened about a month after returning to Boston from a six-month visit to Dagestan last year, Tsarnaev shared the propaganda videos of several radical Islamists, including an insurgent leader who goes by the nom de guerre Abu Dudzhan and hails from the town of Kizilyurt in central Dagestan. “It is just as likely that he was converted [to radical Islam] online as on Kotrova Street,” says the source in Makhachkala. (Some reports claim Abu Dudzhan was killed in a firefight last year, though reports of a militant’s demise in the North Caucasus are all too often premature.)

Tsarnaev’s apparent choice to attend services on Kotrova Street seems to have been part of his religious divergence from his family. Although his mother has said she also became more devout in recent years, the security source in Makhachkala said she was never seen at the Kotrova Street mosque, which generally holds services for men only. The family’s neighbors in Dagestan told TIME over the weekend that Tsarnaev’s father, Anzor, attended services at the more moderate main mosque in Makhachkala, on Dakhadaev Street.

The mosque on Kotrova Street has been one of Russia’s most enduring outposts of Salafi Islam, whose adherents around the world call for strict Shari‘a to govern their societies. Before its construction, smaller Salafi mosques were regularly closed down for extremism in Russia’s predominantly Muslim region of the North Caucasus, which includes Dagestan and neighboring Chechnya. “They would chase us out of one place, and we would congregate in another,” says Magomedtagir Temirchiev, a local devotee of Salafi Islam who helped build the mosque on Kotrova Street in 2000. Its construction was led by a local religious leader named Nadirshakh Khachilaev, who was elected to represent Dagestan in the Russian federal parliament, the State Duma, in 1996. Khachilaev was the leader of the Russian Union of Muslims, which the Ministry of Justice in 2002 deemed as an extremist organization, soon after Khachilaev was charged with orchestrating an ambush on a Russian military convoy in Dagestan. Khachilaev denied those charges. But the case never made it to trial, because Khachilaev was killed in a drive-by shooting outside his home in Makhachkala the following August.

The mosque on Kotrova Street remains at the core of his legacy. It is known in Dagestan either as the Khachilaev Mosque or the Laksky Mosque, after Khachilaev’s ethnic group, the Laks. He is treated as a martyr by many of its congregants, some of whom have also carried on his tendency toward confrontations with the Russian state. “I would be lying if I told you that everyone who gathers there is an angel,” says Temirchiev, one of the usual suspects for counterterrorism forces in Dagestan. “Whenever something blows up, they drag me in for questioning,” he says, his long grey beard making him look much older than his 46 years.

One of the regulars at the mosque on Kotrova Street was Murad Lakhiyanov, one of the most famous leaders of the Islamist underground in Dagestan. In October 2005, police cornered him in a Makhachkala apartment, and after an eight-hour gun battle that included mortar fire from both sides, he was killed. By then, the mosque had already gained infamy as a haunt for local terrorists. In 2002, an explosion ripped through a May Day military parade in the Dagestani town of Kaspiysk, killing 44 people, including 12 children, and wounding 133 others. A manhunt then began for a handful of suspects, some of whom turned out to be regulars at the mosque on Kotrova Street.

Six months later, one of the suspects, Murad Abdurazakov, was found hiding in Temirchiev’s home in Makhachkala. Temirchiev, who spoke to TIME on the terrace of a Makhachkala café just down the street from the mosque he helped build, was sentenced in 2003 for abetting terrorism and served the next six years in the Shamkhal Colony, a high-security prison in Dagestan where many convicted terrorists are held. After his release, he spent time in Moscow and St. Petersburg before returning to Dagestan in the fall of 2012, so he would not have been there at the same time as Tsarnaev. “But I know that our numbers have grown in proportion to the pressure against us,” he says. “Inshallah, they will continue to grow.”

These days, the mosque on Kotrova Street is being expanded. Young men with long beards and skullcaps, the typical accessories of Salafis in Dagestan, have been busy paving the walkway to a new wing of the mosque that is currently under construction. None of the young men working on the project over the weekend said they had ever seen Tsarnaev, but they were not particularly shocked by what he allegedly did in Boston. “Look at what Americans have done in Afghanistan and Iraq,” said one of the men, who would only reveal his first name, Abdullah. “Muslims around the world need to defend each other. That is our belief. So first look at all the Muslim women and children America has killed around the world, and then think about what happened in Boston.”